Bubbles in aerobic activated sludge tanks

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I recently came across a wastewater treatment project for treating acrylonitrile wastewater, using the O/A process. It turns out that the DO in tank O is kept between 0.5 and 1.5 mg/L, while the DO in tank A is kept below 0.5 mg/L. It was found that the sludge in tank O and tank A did not settle; there was a layer of scum on the surface of tank A, and bubbles were present in the MLSS of tank O, with more bubbles in the MLSS of tank A. Due to problems with the aerators, the DO at the outlet of tank O is very low, remaining below 0.5 mg/L most of the time. Since the TN level in this water is around 150 mg/L, it is suspected that the presence of bubbles in the sludge may be caused by denitrification due to too low a DO level in tank O. The DO level in tank O was adjusted, and now it is above 2 mg/L before the water from that tank reaches the sedimentation area; this has been the case for 3–4 days now, yet the bubbles have not disappeared. Another issue is that, despite the system having been in operation for over a year, sludge has not been removed; the sludge appears sticky, and the flocs formed after adding flocculants resemble cotton. Could the presence of bubbles in the sludge be a result of its aging? Sludge removal is now underway, with efforts being made to control the MLSS concentration. I would appreciate it if experts could analyze what causes these bubbles to get trapped in the sludge, and what methods can be used to resolve this issue. Thank you!